!con #15496 Make entries of shards persistent

- Move all entry related logic out of the ShardRegion and into a
  new dedicated child `Shard` actor.
- Shard actor persists entry started and passivated messages.
- Non passivated entries get restarted on termination.
- Shard Coordinator restarts shards on other regions upon region failure or handoff
- Ensures shard rebalance restarts shards.

Refs #15496 
My thoughts where to stop a node when not handing off, was to use the `Passivate` message. Throwing an `IllegalStateException` is fine, but that would cause a restart of the `ShardRegion`. Is that the desired behaviour, when in this case it would have been the `Shard` which had a problem and needed restarting?